The present invention relates to a vessel wall monitoring apparatus which, in the case where a vessel wall is modeled in a mechanical impedance model, are used for monitoring mechanical characteristics of the vessel wall, such as the viscosity, the stiffness, and the inertia.
A vessel plays an important role in carrying oxygen, nutrients, and the like to the whole body in human life support, and changes its state such as dilation and contraction to adjust the blood volume or the like. Such state changes of a vessel are roughly classified into a functional change and an organic change.
A functional change means that peripheral circulation of the human body adjusts dilation and contraction in accordance with a stimulus to the human body. The functional change is produced by a phenomenon that a sympathetic nerve of the autonomic nervous system reacts to a stimulus from the outside of the human body, and the reaction of sympathetic nerve causes a vessel to contract and relax.
During surgery on the human body, therefore, the peripheral circulation is susceptible. In order to know a state change of the peripheral circulation, it is important to monitor mechanical characteristics of a vessel.
By contrast, an organic change means that collagen in the artery wall is metamorphosed and hardened with age or the like and elastic fibers are reduced. In the organic change, organic degeneration of the hardness of the vessel wall is known as a symptom which is called arteriosclerosis.
As described above, in vessel disease, it is highly needed to monitor mechanical characteristics of a vessel wall ranging from short-term monitoring of a patient to chronic disease such as arteriosclerosis, thereby knowing the state of the vessel wall.
In response to such a request, a measuring apparatus in which a vessel wall is modeled in a mechanical impedance model, and factors of the vessel wall such as the inertia, the viscosity, and the stiffness are output has been proposed (see Non-patent Reference 1 and Patent Reference 1). According to the apparatus, based on an electrocardiogram and blood pressure of the patient, and a plethysmogram of blood flowing through a vessel, it is possible to output the above-described mechanical characteristics of the vessel wall, i.e., the inertia, the viscosity, and the stiffness. When the apparatus is used on a patient under surgery, mechanical characteristics of a vessel wall of the patient such as the inertia, the viscosity, and the stiffness can be monitored and the state of the vessel wall of the patient can be known.
It has been experimentally ascertained that the relationship between the blood pressure and the vessel diameter exhibits non-linearity. By contrast, in the apparatus disclosed in Non-patent Reference 1 above, it is assumed that the relationship exhibits linearity (see Exp. 1 of Non-patent Reference 1), and the position is taken that the dependence on the blood pressure itself is information. However, in order to know the degree of tension of the vessel wall due to autonomic nervous activity, and an organic change of the vessel wall due to arteriosclerosis, the high blood pressure dependency with respect to an estimated stiffness value is problematic.
With respect to the problem of the blood pressure dependency, Hayashi et al experimentally show that, when logarithm is applied to the term of the intravascular pressure, the relationship between the human vessel diameter and the intravascular pressure can be linearized, and propose the stiffness parameter, β (hereinafter, this parameter is indicated by βsf, and the stiffness obtained in the invention is indicated by β) as an evaluation index of the vessel elasticity (see Non-patent Reference 2).
In the index, however, only the stiffness of a vessel wall is considered, and only the maximal and minimal blood pressures and the maximum and minimum vessel diameters are used in the estimation. Therefore, it is difficult to evaluate in detail vessel mechanical characteristics. Moreover, also an apparatus which estimates the stiffness and viscosity of a vessel wall based on the maximal and minimal blood pressures and the maximum and minimum vessel diameters is known (see Patent Reference 1).    [Patent Reference 1] JP-A-2006-129958    [Patent Reference 2] JP-A-2008-61910    [Non-patent Reference 1] Monitoring of Vascular Conditions Using Plethysmogram, Akira SAKANE, Toshio TSUJI, Yoshiyuki TANAKA, Noboru SAEKI, and Masashi KAWAMOTO, The journal of the Society of Instrument and Control Engineering, Vol. 40, No. 12, pp. 1236-1242, 2004    [Non-patent Reference 2] Kosaburo HAYASHI, Biomechanics, P73, 2000, Corona Publishing Co., Ltd